1. Field of the Invention
The present invention relates to an apparatus and a method for forming a digital image with use of light intensity modulation in an electrophotographic process.
2. Description of the Prior Art
In a digital printer and the like with use of an electrophotographic process, a laser beam is modulated according to multi-level image data to expose a photoconductor which has been sensitized uniformly, to form an electrostatic latent image. The latent image is developed with toners, and the resultant toner image is transferred to a paper. Thus, the image data is reproduced on the paper.
In this intensity modulation method, the size of laser beam is kept constant. However, the beam size varies with the type of laser diode used for the exposure. In U.S. Pat. application Ser. No. 07/671,963, filed Mar. 19, 1991 the beam size of laser diode is coded, and the coding is performed in a factory before shipment. The beam size (code input) of a laser diode installed in an apparatus is detected, and the gradation correction data is corrected according to the detected beam size.
A latent image formed with the laser beam is an analog image, and the gradation characteristic depends largely on the size of laser beam. The size of laser beam varies with the environment and the like of the electrophotography process. Therefore, it is desirable that gradation characteristic is corrected more precisely according to the beam size.
The beam size can be detected with a detector disclosed for example in Japanese Patent laid open Publication No. 68,358/1985. Usually, the beam size is detected, and a controller changes the beam size in response to the detected beam size (so-called autofocus). However, if such a detector and a controller are installed in a printer, the cost becomes higher. On the other hand, Japanese Patent laid open for Publication No. 25,164/1986 discloses the control of the spot size of the beam by changing the intensity of the beam. This method can control the spot size with a simple structure. However, it cannot control the gradation of a half-tone image appropriately because the potential of a latent image varies with the intensity of the beam.